The present invention pertains to nuclear reactor fuel assemblies, and more particularly, to a fuel assembly grid of the type having structure dedicated to enhancing the mixing of coolant passing through the grid.
U.S. Pat. No. 4,879,090, "Split Vaned Nuclear Fuel Assembly Grid", issued Nov. 7, 1989 to Perrotti et al, discloses a nuclear fuel assembly grid having integral vanes with optimized size, shape and bend angles for maximizing coolant mixing and fuel rod heat transfer. Several other optimization features are also disclosed in said patent. The grid strips disclosed therein are similar to other flow-mixing grids, however, in that the vanes integrally project from the upper edge of each strip, generally toward the fuel rod. Despite the improved performance of the grid described in said patent relative to the grids available at the time the invention described therein was made, several fabrication and performance characteristics were identified as warranting further improvement.
One such area of desired improvement pertains to the inconvenience during fabrication, of making a joining weld between the intersecting strips, at the top of the intersection, in substantially the same location where the vanes project upwardly, e.g., the vanes tend to hide the upper strip intersection weld location. The second fabrication-related desire for improvement, arises from the extra care that must be taken when initially inserting fuel rods into the grid, and especially during fuel assembly reconstitution, to avoid damaging the cantilevered, projecting vanes.
With respect to desired performance improvements, it has been recognized that the presence of the upper strip intersection weld at the base of the mixing vanes, adversely affects the coolant flow immediately upstream of the vane, thereby reducing the vane effectiveness to some extent. Also, the cantilevering of the vane upwardly from the upper edge of the strip, permits leakage of some of the redirected coolant laterally of the longitudinal orientation of the vane, thereby also reducing the effectiveness of the desired mixing.